The recovery and utilization of natural gas and other methane-rich gas streams as economic fuel sources have required the liquefaction of the gas in order to provide economic transportation of the gas from the site of production to the site of use. Liquefaction of large volumes of gas is obviously energy intensive. In order for natural gas to be available at competitive prices the liquefaction process must be as energy efficient as possible.
Inefficiencies in liquefaction processes are usually present when the compression load on the refrigeration equipment used to perform the liquefaction is not balanced on the drivers or electric motors used to run the equipment in a single component refrigerant cycle, specifically when such equipment is matched throughout the liquefaction installation. Compression load is the major power comsuming function of a liquefaction process. In addition, a liquefaction process must be readily adaptable to varying regions with their specific climatic conditions. Such climatic conditions may also vary seasonally particularly in the more polar extreme regions of the world. Such climatic conditions affect a liquefaction process predominantly in the temperature of the cooling water utilized in the production of refrigeration used to liquefy the natural gas. The sizeable variations in the temperature of available cooling water due to changing seasons or different climatic zones can cause imbalances in the various refrigeration cycles.
Other inefficiencies may also arise aside from the matching of compression load with compression drivers in the refrigeration cycles. Such inefficiencies usually reside in the matching of gas to be liquefied against refrigerant to perform the liquefaction. For a multicomponent stage flash cycle, compositional variations and constraints have plagued those skilled in the art.
Various attempts have been made to provide efficient liquefaction processes, which are readily adaptable to varying ambient conditions and multiple component, multiple cycle refrigerant processes. In U.S. Pat. No. 4,112,700 a liquefaction scheme for processing natural gas is set forth wherein two closed cycle refrigerant streams are utilized to liquefy natural gas. A first high level (higher temperature) precool refrigerant cycle is utilized in multiple stages to cool the natural gas. The refrigerant is not initially condensed totally against cooling water. This first high level precool refrigerant is phase separated in multiple stages, wherein the effect is to return the light component portions of the refrigerant for recycle, while the heavy component portions of the refrigerant are retained to perform the cooling at lower temperatures of the natural gas. The first high level precool refrigerant is also utilized to cool the second low level (lower temperature) refrigerant. The second low level refrigerant performs the liquefaction of the natural gas in a single stage. The drawback in this process is that the high level precool refrigerant after initial phase separation utilizes heavier and heavier molecular weight components to do lower and lower temperature cooling duty. This is contrary to the desired manner of efficient cooling of the present invention. Further, the second or low level refrigerant is used in a single stage to liquefy the natural gas, rather than performing such liquefaction in multiple stages. Finally, the high level refrigerant is not totally condensed against external cooling fluid prior to its refrigeration duty.
U.S. Pat. No. 4,274,849 discloses a process for liquefying a gas rich in methane, wherein the process utilizes two separate refrigeration cycles. Each cycle utilizes a multicomponent refrigerant. The low level (lower temperature) refrigerant cools and liquefies the natural gas in two stages by indirect heat exchange. The high level (higher temperature) refrigerant does not heat exchange with the natural gas to be liquefied, but cools the low level refrigerant by indirect heat exchange in an auxiliary heat exchanger. This heat exchange is performed in a single stage.
U.S. Pat. No. 4,339,253 discloses a dual refrigerant liquefaction process for natural gas, wherein a low level refrigerant cools and liquefies natural gas in two stages. This low level refrigerant is, in turn, cooled by a high level refrigerant in a single stage. The high level refrigerant is used to initially cool the natural gas only to a temperature to remove moisture therefrom before feeding the dry natural gas to the main liquefaction area of the process. The use of such individual stage heat exchange between the cycles of a dual cycle refrigerant liquefaction process precludes the opportunity to provide closely matched heat exchange between the cycles by the systematic variation of the refrigerant compositions when the refrigerants constitute mixed component refrigerant.
In the literature article Paradowski, H. and Squera, O., "Liquefaction of the Associated Gases", Seventh International Conference on LNG, May 15-19, 1983, a liquefaction scheme is shown in FIG. 3 wherein two closed refrigeration cycles are used to liquefy a gas. The high level cycle depicted at the right of the flowscheme is used to cool the low level cycle as well as cool for moisture condensation an initial gas stream. The high level refrigerant is recompressed in multiple stages and cools the low level refrigerant in three distinct temperature and pressure stages. Alteration of the high level refrigerant composition to match the various stages of refrigeration in the heat exchanger is not contemplated.
The present invention overcomes the drawbacks of the prior art by utilizing a unique flowscheme in a liquefaction process utilizing two multiple component refrigerants in closed cycles, where the refrigerants are heat exchanged one with another in multiple stages while the refrigerant composition of the high level refrigerant is varied such that lighter molecular weight components of the refrigerant are available to perform the lower level (low temperature) refrigeration duty which is best suited to such lower molecular weight components.